Fix poor handling of nullptr for new blur code

BUG=chromium:749147

Change-Id: I2d4259805300c3f5ed9d25e142e9f12570c59781
Reviewed-on: https://skia-review.googlesource.com/37121
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
3 files changed